Daily activity in old age can be regarded as a vital sign. Measures of endurance, such as a 6-minute walk test, and measures of function, such as a 6-meter walk, are predictive of mortality and morbidity and measures like this are included in the REHAB-HF Trial. However, both of these tests are limited as they are performed within a clinical setting. Accelerometry data can be used to monitor physical activity in everyday life, including hours normally reserved for sleep. These data yield information about the volume, duration, intensity and distribution of activity to predict health outcomes. Clinical application of monitoring for chronic disease is just beginning and there are no other studies with continuous monitoring of acute decompensated heart failure (ADHF) patients. We propose continuous objective monitoring in the REHAB-HF Study to address hypotheses about the effect of the intervention and to provide information about subclinical change in activity predictive of ADHF events.